Power semiconductor devices, such as power diodes, power MOSFETs (metal oxide semiconductor field effect transistors), power IGBTs (insulated gate bipolar transistors) or power thyristors, are designed to withstand high blocking voltages. Those power devices include a pn-junction that is formed between a p-doped semiconductor region and an n-doped semiconductor region. The component blocks (is switched off) when the pn-junction is reverse biased. In this case a depletion regions or space charge zone propagates in the p-doped and n-doped regions. Usually one of these semiconductor regions is more lightly doped than the other one of these semiconductor regions, so that the depletion region mainly extends in the more lightly doped region, which mainly supports the voltage applied across the pn-junction. The semiconductor region supporting the blocking voltage is referred to as base region in a diode or thyristor, and is referred to as drift region in a MOSFET or an IGBT.
The ability of a pn-junction to support high voltages is limited by the avalanche breakdown phenomenon. As a voltage applied across a pn-junction increases, an electric field in the semiconductor regions forming the pn-junction increases. The electric field results in an acceleration of mobile charge carriers present in the semiconductor region. An avalanche breakdown occurs when, due to the electric field, the charge carriers are accelerated such that they create electron-hole pairs by impact ionization. Charge carriers created by impact ionization create new charge carriers, so that there is a multiplication effect. At the onset of an avalanche breakdown a significant current flows across the pn-junction in the reverse direction. The voltage at which the avalanche breakdown sets in is referred to as breakdown voltage.
The electric field at which the avalanche breakdown sets in is referred to as critical electric field (Ecrit). The absolute value of the critical electric field is mainly dependent on the type of semiconductor material used for forming the pn-junction, and is weakly dependent on the doping concentration of the more lightly doped semiconductor region.
The critical electric field is a theoretical value that is defined for a semiconductor region that has an infinite size in directions perpendicular to field strength vectors of the electric field. Power semiconductor components, however, have semiconductor bodies of finite size that are terminated by edge surfaces in lateral directions. In vertical power semiconductor devices, which are semiconductor devices in which the pn-junction mainly extends in a horizontal plane of the semiconductor body, the pn-junction usually does not extend to the edge surface of the semiconductor body but is distant to the edge surface of the semiconductor body in a lateral direction. In this case, a semiconductor region (edge region) of the semiconductor body adjoining the pn junction in the lateral direction also has to withstand the blocking voltage.
The edge region could be implemented as a doped region with the same doping concentration as the base or drift region. In this case, however, the dimension of the edge region in the lateral direction of the semiconductor body is at least the dimension (length) of the drift region in the vertical direction. The length of the drift region can be up to several 10 micrometers (μm) and more, dependent on the desired voltage blocking capability, so that a corresponding edge termination would be very space consuming.
In order to reduce the space required for withstanding the blocking voltage in the edge region, an edge termination with a vertical dielectric layer arranged in a trench can be provided. In order to support high voltages, a thick dielectric layer is required. A thick dielectric layer in a trench, however, can cause mechanical stress in the semiconductor body.
There is therefore a need for a semiconductor device with an efficient and space saving edge termination.